1. Field of the Invention
The invention is directed to an apparatus for reducing PCDD and PCDF emissions from SCR catalyzers which reduce nitric oxides in the exhaust gas of the internal combustion engine by means of ammonia.
2. Description of the Related Art
Nitric oxides are some of the limited components of exhaust gas which are formed during combustion processes. Permissible emissions of these components with respect to the environment continue to be lowered. Reduction of nitric oxides is usually accomplished by means of catalyzers; reducing agents are additionally required in oxygen-rich exhaust to increase selectivity and NOX conversion. This method is referred to as selective catalytic reduction, or SCR, and has been used in the energy industry since 1980. V2O5-containing mixed oxides, e.g., in the form of V2O5/WO3/TiO2, can be used as SCR catalyzers. V2O5 proportions typically range between 0.2% and 3%. Ammonia or compounds which split off ammonia such as urea or ammonia formiate are used in solid form or in solution as reducing agents. The reaction proceeds as follows:4NO+4NH3+O24N2+6H2O   (1)
Special problems arise when using the SCR method to reduce nitric oxides in internal combustion engines, especially in vehicles, because emission of unspent ammonia must be prevented. Unlike in the energy industry, there are no sufficiently accurate, durable exhaust gas sensors available for regulating the system in vehicles and, therefore, for preventing NH3 emissions in the event of excessive dosing.
Particle separators, as they are called, or particle filters are used in the energy industry and in vehicles to minimize fine particles.
A typical arrangement with particle separators for use in vehicles is described, for example, in EP 1 072 765 A1. Arrangements of this kind differ from those using particle filters in that the diameter of the channels in the particle separator is substantially greater than the diameter of the largest occurring particle, while the diameter of the filter channels in particle filters is in the range of the diameter of the particles. Due to this difference, particle filters are subject to blockage, which increases the exhaust gas counterpressure and lowers engine performance. An arrangement and a method with particle filters are shown in U.S. Pat. No. 4,902,487. A distinguishing feature of the two above-mentioned arrangements and methods is that the oxidation catalyzer—usually a catalyzer with platinum as active material—arranged upstream of the particle separator or particle filter oxidizes the nitrogen monoxide in the exhaust gas to form nitrogen dioxide by means of the residual oxygen which is also contained.2NO+O22NO2   (2)
In this regard, it must be ensured that the equilibrium of the above reaction lies on the side of NO at high temperatures. As a result, the achievable NO2 proportions are limited at high temperatures due to this thermodynamic limitation.NO+2NH3+NO22N2+3H2O   (3)
This NO2 is in turn converted in the particle separator or particle filter with the carbon particles to form CO, CO2, N2 and NO.
There is a continuous removal of the deposited fine particles by means of the powerful oxidizing agent NO2, so that regeneration cycles such as those which must be laboriously carried out in other arrangements are dispensed with. For this reason, this is referred to as “passive” regeneration.2 NO2+C2 NO+CO2   (3)NO2+CNO+CO   (4)2 C+2 NO2N2+2CO2   (5)
If the NO2 does not succeed in effecting a complete oxidation of the carbon embedded in the particle filter, the carbon proportion and, therefore, the exhaust gas counterpressure increases steadily.
At the present time, this is prevented by providing the particle filters with a catalytic coating for the oxidation of NO. As was already described above, these catalyzers usually contain platinum. The disadvantage of this method is that the NO2 formed at the particle filter can only be used for oxidation of particles which have been separated out downstream of the catalytically active layer for NO oxidation, that is, inside the filter medium. However, if a layer of separated particles, or a filter cake as it is called, should form on the filter surface and, therefore, on the catalytically active layer, the NO oxidation catalyzer lies downstream of the filter cake so that the soot particles separated out at that location cannot be oxidized by means of NO2 from the NO oxidation catalyzer arranged on the particle filter.
In addition, only the catalyzer layer arranged on the raw gas side contributes, strictly speaking, to the performance of the system because the NO2 that is formed catalytically on the purified gas side can no longer come into contact with the soot deposited on the raw gas side and inside the filter material.
Another problem arising from the coating of the particle filter is that the geometric surfaces of the filter are appreciably smaller than those of the catalyzer substrates that are normally used. The reason for this is that the filters require relatively large free cross sections and, therefore, free volume on the raw gas side so that soot and engine oil ashes can be embedded. When ceramic filter substrates are used, this is implemented by means of a low porosity of 50 cpsi to 200 cpsi. On the other hand, simple catalyzers are usually constructed with cell densities of 400 cpsi to 900 cpsi. An increase from 50 cpsi to 900 cpsi results in an increase in the geometric surface from 1 m2/l to 4 m2/l, which makes possible substantially increased throughputs at the catalyzers.
For these reasons, an NO oxidation catalyzer cannot be omitted in front of the particle filter in spite of the catalytic coating of the filter. This leads to a relatively large structural volume. This is the case even when the NO oxidation catalyzer and particle filters form a constructional unit by constructing the input area of the particle filter as an NO oxidation catalyzer (DE10327030 A1).
Although these steps allow soot oxidation up to temperatures of 250° C., there are applications in which even these exhaust gas temperatures cannot be reached and, therefore, reliable functioning of the particle filters cannot be ensured. This normally occurs in lightly loaded engines installed in vehicles, for example, in passenger cars, public buses, and garbage collection trucks, which, moreover, also have high idling proportions.
Therefore, a second possibility for particle filter regeneration is applied especially in these cases: this consists in actively raising the exhaust gas temperature. Usually this is accomplished by adding hydrocarbons upstream of oxidation catalyzers. The exothermal oxidation of the hydrocarbons at the catalyzers leads to an appreciable rise in temperature.
When the temperature is increased to more than 600° C. in this way, the carbon is oxidized by means of oxygen.C+O2CO2   (6)
However, the risk in this so-called “active” filter regeneration is that the burning of the soot will lead to a sharp uncontrolled rise in temperature of up to 1000° C. and, therefore, usually to damage to the particle filter and/or catalyzers arranged downstream.
Since the temperature increase must be maintained for several minutes to ensure a quantitative oxidation of the soot particles, the need for hydrocarbons is significant and, because the fuel in the internal combustion engine is usually used as a source of hydrocarbons, its efficiency is impaired.
The addition of hydrocarbons can be carried out by means of a separate injection nozzle arranged in the exhaust system. Another possibility is to generate high hydrocarbon emissions by means of a delayed after-injection of fuel into the combustion chamber.
In order to meet future exhaust gas regulations, it will be necessary to use arrangements for reducing nitric oxide emissions and arrangements for reducing fine particles emissions at the same time.
One solution is to coat the particle filter with SCR-active material (JP 2004-376102). In this connection, the use of V2O5 as an SCR-active component is difficult. This is due to the poor thermal stability of these catalyzers. Exhaust gas temperatures of more than 650° C. lead to sublimation of V2O5. Since these temperatures can easily occur in particle filters, as was already mentioned above, V2O5-free catalyzers containing transition metals, especially iron-, cobalt-, or copper-containing catalyzers, are used for these and other high-temperature applications. The integration of these transition metals through ion exchange in zeolites has proven to be particularly advantageous in this connection (U.S. Pat. No. 5,017,538). In this way, because of the very large surface of the zeolites, it is possible to substantially enlarge the active surface and accordingly appreciably increase the achievable throughput.
The disadvantage of these transition metal-containing catalyzers, however, is that they form highly toxic polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) in the presence of chloride and hydrocarbons in the temperature range between 200° C. and 400° C.
In the vehicle, the chloride needed for dioxin formation reaches the exhaust gas and, accordingly, the catalyzers, e.g., through biofuels, the engine oil, or the intake air (salt spraying in winter, driving in coastal regions). The hydrocarbons needed for the formation of PCDD and PCDF are contained in the exhaust gas in any case because of incomplete combustion of the fuel.